Etude du mélange $B^{0}_s- \overline{B^{0}_s}$ dans l'expérience ALEPH au LEP

A search for $B^0_s$ oscillations is performed in a sample of semileptonic $B$ hadron decays collected by the ALEPH experiment during 1991--1995. Compared to previous inclusive analyses, the proper time resolution and mesons flavour tagging are significantly improved. In addition, subsamples of the data are enriched in $B^0_s$. Unbinned maximum likelihood fits are performed to derive: \begin{center} $ \Delta m_s \gt 9.4$ ps$^{-1}$ at 95\% confidence level, \end{center} which is the highest limit to date. Combining with the ALEPH $D^-_s$ based analyses yields $\Delta m_s \gt 9.5$ ps$^{-1}$ at 95\% confidence level. This provides a significant constraint on the right side of the Unitarity Triangle of the CKM matrix.